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Motion changes due to an interaction between two objects
In the Aristotelian theory, natural motion and voluntary motion are
one-sided phenomena: the object causes its own motion. Forced motion is
supposed to be a two-sided phenomenon, because one object imposes its
“commands” on another. Where Aristotle conceived of some of the phe-
nomena of motion as one-sided and others as two-sided, Newton realized
that a change in motion was always a two-sided relationship of a force
acting between two physical objects.
The one-sided “natural motion” description of falling makes a crucial
omission. The acceleration of a falling object is not caused by its own
“natural” tendencies but by an attractive force between it and the planet
Earth. Moon rocks brought back to our planet do not “want” to fly back up
to the moon because the moon is their “natural” place. They fall to the floor
when you drop them, just like our homegrown rocks. As we’ll discuss in
more detail later in this course, gravitational forces are simply an attraction
that occurs between any two physical objects. Minute gravitational forces
can even be measured between human-scale objects in the laboratory.
The idea of natural motion also explains incorrectly why things come to
rest. A basketball rolling across a beach slows to a stop because it is interact-
ing with the sand via a frictional force, not because of its own desire to be at
rest. If it was on a frictionless surface, it would never slow down. Many of
Aristotle’s mistakes stemmed from his failure to recognize friction as a force.
The concept of voluntary motion is equally flawed. You may have been
a little uneasy about it from the start, because it assumes a clear distinction
between living and nonliving things. Today, however, we are used to having
the human body likened to a complex machine. In the modern world-view,
the border between the living and the inanimate is a fuzzy no-man’s land
inhabited by viruses, prions, and silicon chips. Furthermore, Aristotle’s
statement that you can take a step forward “because you choose to” inap-
propriately mixes two levels of explanation. At the physical level of explana-
tion, the reason your body steps forward is because of a frictional force
acting between your foot and the floor. If the floor was covered with a
puddle of oil, no amount of “choosing to” would enable you to take a
graceful stride forward.
Forces can all be measured on the same numerical scale
In the Aristotelian-scholastic tradition, the description of motion as
natural, voluntary, or forced was only the broadest level of classification, like
splitting animals into birds, reptiles, mammals, and amphibians. There
might be thousands of types of motion, each of which would follow its own
rules. Newton’s realization that all changes in motion were caused by two-
sided interactions made it seem that the phenomena might have more in
common than had been apparent. In the Newtonian description, there is
only one cause for a change in motion, which we call force. Forces may be
of different types, but they all produce changes in motion according to the
same rules. Any acceleration that can be produced by a magnetic force can
equally well be produced by an appropriately controlled stream of water. We
can speak of two forces as being equal if they produce the same change in
motion when applied in the same situation, which means that they pushed
or pulled equally hard in the same direction.
“Our eyes receive blue light reflected
from this painting because Monet
wanted to represent water with the
color blue.” This is a valid statement
at one level of explanation, but physics
works at the physical level of
explanation, in which blue light gets
to your eyes because it is reflected by
blue pigments in the paint.
Chapter 4Force and Motion